Article for preventing edge defect in compression stretched acrylic sheet

ABSTRACT

An article for preventing edge defects in compression-stretched acrylic sheets comprising providing the circumferential end surfaces of an acrylic blank to be stretched with a groove substantially continuous therewith, the depth of the groove being greater than 5 percent and, preferably, being greater than about 10 percent of the initial thickness of the acrylic blank. The groove may have various configurations, e.g., V- and U-shaped configurations. Preferably, the center line of the groove lies in the plane bisecting the blank in the thickness direction.

United States Patent 1 a June 6, 1972 Ayres [54] ARTICLE FOR PREVENTINGEDGE DEFECT IN-COMPRESSION STRETCHED ACRYLIC SHEET [72] Inventor: RonaldL. Ayres, Saugus, Calif.

[73] Assignee: For-tin Plastics, Inc., Saugus, Calif. '22 Filed: May 25,1970 21 App]. No.: 48,691

Related US. Application Data [62] Division of Ser. No. 758,395, Sept. 9,1968, Pat. No.

[52]- U.s.c|.. ...l6l/1l8,l6l/ll7,161/149 [51] lnt.Cl...B32b3/02,B32b-3/30 Y 58 FieldofSearch ..161/44,ll7,118,l24, 149,

[56] References Cited UNITED STATES PATENTS 2,201,669 5/1940 Kraft..l6l/l49 X 3,262,136 7/1966 Sevcik.... .....l6l/ll8 X PrimaryExaminer-William A. Powell Attorney-Fulwider, Patton, Rieber, Lee &Utecht [57] ABSTRACT 4 Claim, 1 1 Drawing Figures stretched acrylicpart.

" CROSS-REFERENCES TQRELATED APPLICATION This application is adivisional application of US. Pat. application-Ser. No. 758,395, filedSept. 9, 1968, now US. Pat. No. 3,562,383. a I

- 1 BACKGROUND OF THE INVENTION This invention relates to'stretching ofacrylic sheets and, more particularly, to the prevention of edge defectsin stretched acrylic sheets.

Because of its strength, formability, and light transmissioncharacteristics, acylic material has a variety of uses and particularlyhas been used to form'windows and canopies for-aircraft. Manufacture ofthe acrylic part often involves a stretching step in which the acrylicmaterial is heated and stretched to enhance its physical characteristicsand/or to provide the acrylic part with a required curvature.Heretofore, stretching of acrylic blanks has been accomplished by eitherpulling-the edges of the acrylic blanks in opposing directions such asis describedin US. Pat. No. Re. 24,978 of Bottoms et al., ,or ofj'cornpression stretching acrylic blanks as described in us. Pat.application Ser. No.69 1 ,294 ofTerry D. Fortin, filed on Dec. 18,1967,now abandoned. Regardless of the way-in which the acrylic blanks arestretched, stresses are set up in the edges of the acrylic blanks whichcause cracking and subsequent loss of material around the edges of theacrylic blanks. Additionally, such edge cracks can propogate throughoutthe stretched sheet, thereby resulting in a near total loss of theacrylic part.

To reduce losses from acrylic blanks stretched by the tension methoddescribed in the above-mentioned Bottoms patent, the edges of theacrylic blanks have been cooled relative to the remainder of the acrylicblank-as the acrylic blank is stretched. This method has reduced thematerial lost when stretching by .this "method. However, other stressesare produced in the stretched acrylic sheet due to the temperaturedifferential between the edge and interior sections of the To the bestof the knowledge of the instant inventor, no methods have beenpreviously used to prevent edge losses when stretching acrylic blanks bythe compression method described in the aforementioned Fortinapplication.

I SUMMARY OF THE INVENTION Themethod of this invention comprisesproviding the circumferential end surfaces of acrylic blanks with asubstantially continuous groove therein. The depth of the groove is atleast about 5 percent and, preferably, is greater than about percent ofthe initial thickness of the acrylic blank. The groove may have avariety of configurations such as, for example, V- and U-shapedconfigurations and the centerline of the groove may be offset from thecenter-line of the circumferential end surfaces. However, it ispreferable to position the groove so that its center-line correspondswith the center-line of the circumferential end surfaces.

The grooving of acrylic blanks as described is relatively easy toaccomplish and the machining producing the groove ap-' parently sets upno noticeable stresses in the edge of the acrylic blanks. In'spite ofits simplicity and ease of accomplishment,

the method of this invention has been extremely successful insubstantially eliminating lossesfrom the edges of compression stretchedacrylic sheets.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 3 is a partial perspective viewvof a compressionstretched acrylic sheet stretched from the acrylic blankof FIG. 1;

FIG. 4 is a partial sectional view of the unstretched acrylic blank ofFIG. 1 taken along the lines 4-4 of FIG. 1;

FIG. 4a is a sectional view of the stretched acrylic blank of FIG. 3taken along the lines 4a 4dof FIG. 3;

FIG. 5 is a sectional view of an acrylic blank provided with anotherform of circumferential-groove of this invention but different in shapefrom the groove of FIG. 4;

FIG. 5a is a sectional view of that portion of the acrylic blank shownin FIG. 5 after being compression-stretched;

FIGS. 6, 7 and 8 are sectional views of unstretched acrylic blankshaving variously shaped and positioned circumferential end grooves ofthis invention; and. v FIG. 9 is a partial elevational view ofastretched acrylic sheet showing edge roll" produced' from compressionstretching. I

DESCRIPTION OF THE PREFERRED EMBODIMENT The method of this'inventioncomprises' providing the 'circumferential end surfaces ofacrylic blanks with a groove which is substantially continuoustherewith. The depth of the groove is at least about 5 percent and,preferably is at least aboutlO percent of the initial thickness of theacrylic blank. Various configurations, e.g., V- and U-shaped andrectangular configurations, may be used. Preferably, thecenter line ofthe groove lies along the center line of the acrylicjblank taken in thethickness direction. I

Acrylic blanks having the aforedescribed. groovemay be heated andcompression stretched as described in the US. application Ser. No.691,294 ofTerry-D. Fortin, filed Dec. 18, 1967 (hereafter referred. toas said Fortin application"). When acrylic blanks are grooved stretchedin this manner, the resultant stretched acrylic sheetsexhibitsubstantially no edge defects. Therefore, the loss of acrylic materialis substantially eliminated. In acrylic blanks which are not providedwith the groove of this invention, the edges of the acrylic blanks,after'being stretched by the method described in said Fortinapplication, often are highly stressed resulting in weakened or crackededges (FIG. 2).

Acrylic blanks'stretched according to said Fortin application'. areheated to their soflening' temperature range and, thereafter, compressedto cause the acrylic material to flow or squeeze radially outwardly toform a stretched acrylic sheet of reduced thickness. It is believedthatthe undesirable edge defects. result from the partially molten edgesof the acrylic blanks rolling" non-uniformly outwardly from the centralportion of the acrylic blanks. This "frolling" of the outer edgesapparently takes place at a rate faster than the radially outwardmovement of the remainder of the acrylic blank. This difi'erential rateof movement is believed to be the cause of the stresses and cracks whichare introduced into the edges of the stretched acrylic sheet. Thesuccess of the groove of this invention in preventing fracturing of theedges of compression stretched acrylic sheet is believed due to therestraining of this "edge roll" due to a change in edge forces which is,in some manner, derived from the aforedescribed circumferentialgrooving. That is, it is hypothesized that the initial compressionforces exerted on the groove exceed the non-uniform outward forces whichtend to create edge roll, thereby restraining outward flow of the edgematerial so that its outward flow is substantially equal to the outward'flow of the remainder of the acrylic blank as the blank is stretched.

The method of this invention will now be more particularly describedwith reference to the figures. In FIG. 1, the numeral 10 designates anas-cast acrylic blank which has a circumferential end surface 11. Theterm acrylic blank" will be employed herein, and in the claims, todenote unstretched acrylic material. The end surface 11 of the blank 10is provided with a continuous groove 12. Some improvement in the edgesof compression-stretched acrylic sheet may be produced re- 7 gardless ofthe shape and size of the groove 12 in the acrylic which nocircumferential grooves were formed. By com- ,parison, acrylic sheetsstretched'from blanks having circumferentialend grooves of depth greaterthan about 10 percent of the blank thickness, substantially-alwaysexhibit no edge defects-.'.. Y

As used herein, the term edge defects" refers to a random,

uneven stressing, cracking and/or breaking of the edges of stretchedsheets as shown, byway of example, in FIG. 2. Such edge ,defects canresult in substantial loss of acrylic material. While the shallowcircumferential grooves which may be present around the edges of acrylicmaterial after it has been stretched by the herein-described method,such as shown in FIG. 3, and constitutes a loss of material, such lossis only a minor fraction of that due to uneven stresses and other edgecracking. This loss is minimal because the shallow grooves extend auniform distance into the material and are not accompanied by cracking,stressing, etc., which would necessitate the removal of good material inorder to remove stressed or cracked sections as in the case withstretched sheets exhibiting edge defects. i

- A section 13 of an acrylic sheetoriginally having a shape and edgegroove 12 such as is shown in FIG. I, is shown in FIG. 3 after beingcompression-stretched. In addition to havinga reduced thickness ascompared with the blank 10 of FIG.

1, the stretched sheet 13 has a much smaller edge groove 14.

The smaller groove 14 is produced by the filling'in of the originalgroove 12 as the acrylic blank 10 is compressionstretched. The latter isfurther shown by comparing the grooves 12 of FIGS. 4 and with thegrooves 14 of FIGS. 4a and 5a, respectively. As will be seeri from thiscomparison, the stretched sheets 13 of FIGS. 4a and 5a havesubstantially smaller grooves14 than the acrylic blanks of FIGS. 4 and5.

It has also beenfound that the most advantageous results are obtainedwhen the grooves in the circumferential end surfaces of the blanks aresymmetrical about the plane passing through the center-line of suchsurfaces as shown in FIGS. 4, 5

and 6..-That is, optimum results are produced when the maximum depth of.the grooves is located at the center-line of the circumferentialsurfaces and the center-line of the groove corresponds to thecenter-line of the circumferential surfaces.

With thisoptimumpositioning of the grooves in the blanks, the materialwhichhas to be removed from the edge of the stretched sheetto remove anyshallow grooves remaining after compression is minimized, as comparedwith an asymmetrical groove as shown in F 168.7 and 8.

The shape of the groove does not appear to be critical. Grooves ofvarious cross-sectional shapes, e.g., V-shaped, U- shaped, circular andrectangular, (FIGS. 4, 5 and 6) have been successfully employed.Additionally, the mouth of the groove may be as wide as, or narrowerthan, the width of the circumferential end surfaces, as shown in FIGS. 5and 8. The groove is substantially coextensive with the length of thecircumferential end surface (FIG. 1) since edge defects may beintroduced into the stretched sheetat any point along the. end

surface if there is no grooving at that point.

v In actual stretching operations, it is preferable to provide a testblank with a groove having a particular depth (greater than 10 percentof the thickness of the blank) and, thereafter, to compression stretchthe test blank to the'desired stretched size.'By examining the depth ofthe groove in the stretched part, it canjbe determined whether to use,shallower or deeper grooves in the production blanks. For example, if arelatively deep groove remains in the stretched sheet, a shallowergroove than initially present in the test blank can be formed or cut'inthe production blanks so that edge losses from the stretched part willbe further minimizedp Cracking and stressing of theedges is generallypreceded by edgeroll" as shown in FIG. 9. Thus, if edge roll is presentalong the edges of a stretched acrylic sheet, a border-line conwhichwill be substantially eliminated in the stretched acrylic sheet. Thatis, optimally the depth of the acrylic blank groove is selected so thatthe flow of material during compression stretching substantially fillsthe groove. Such groove selection, of course, minimizes or eliminatesany losses of material from the stretched sheet. I

Following'the choice'of a satisfactory. groove depth, the productionacrylic blanks are provided with edge groove by any method, e.g.,sawing, which will not critically stress the blank. The blanks are thenready to be compression stretched. Compression stretching of the groovedblanks may be performed by any desired technique. However, it ispresently preferably to employ the compression stretching methoddescribed in said Fortin application, the description of which isincorporated herein by reference to said F ortin application. Examplesof the article and process of this invention follow:

EXAMPLE 1 An as-cast Plexiglas 55 blank'(manufactured by Rohm & Haas)having initial dimensions of 6 in. X ,6 in. X 0.751. in. thick wasprovided with a circumferential groove having a configurationsubstantially as shown in FIG. 8. The maximum depth of the groove'was0.125 inches, or'about 16 percent of the initial acrylic blankthickness. This blank was coated with Mold-Wiz F-57 lubricant (Tefloncolloidal dispersion manufactured by Axel Plastics Research Lab., Inc.)and placed between a pair of polished glass'plates, and isothermallyheated to a temperature of 295 F. I

The blank was thereafter compressed as a thickness reduction rate ofabout 0.10inches per minute to produce a stretched acrylic sheetmeasuring 9.5in. x 9.5 in. x 0.270 in. thick. Thereafter, thetemperature of the acrylic sheet was reduced at a rate of l3 F. perminute to'a temperature below aboutll0F. r

No cracks developed in the stretched acrylic sheet and no edge roll wasobserved. Additionally, the size of the groove in the blank wassubstantially reduced.

5 EXAMPLE z 4 same conditions as described in Example I to a stretched.

acrylic sheet having dimensionsof 9 in. X 9 in. X 0.350 in. thick. I y

No cracks or edge roll were observed in the stretched acrylic sheet andthedepth of theoriginal groove in the acrylic blank was substantiallyeliminated in the stretched sheet.

EXAMPLE 3 An acrylic blank formed from the same material and having thesame dimensions as that described in Example 2 was provided with aV-shaped groove similar to that used in Example 2 except that themaximum depth of the groove was 0.046 inches or about 6 percent ofthe'acrylic blank thickness. This blank was stretched under the sameconditions as employed in Example 2 to a stretched acrylic sheet havingdimensionsof 9.5 in. X 9.5 in. X 0.300 in. thick.

No cracks were observed in the edges of the stretched acrylic sheet.However, some edge roll was noticed along the edges of the sheetand,from experience, this indicated a borderline condition.

EXAMPLE 4 EXAMPLE 5 An acrylic blank of Plexiglas 55 having dimensionsof 12.5 in. X 12.5 in. X 0.780 in. was provided with a continuouscircumferential groove similar to that shown in FIG. 7 and having amaximum depth of about 0.375 inches (about 50 percent of the initialblank thickness). This blank was compression stretched by the method ofExample 1 to a stretched acrylic sheet having dimensions of 22.2 in. X22.2 in. X 0.233 in.

No cracks or edge roll were observed along the edges of the stretchedacrylic sheet.

EXAMPLE 6 An as-cast acrylic blank of Plexiglas 55 having dimensions of12.5 in. X 12.5 in. X 0.735 in. was provided with a continuouscircumferential V-shaped groove as shown in FIG. 4 and having a maximumdepth of about 0.312 inches (about 40 percent of the initial blankthickness). This acrylic blank was stretched by the method of Example 1to a stretched acrylic sheet having dimensions of 21 in. X 22 in. X0.235 in.

No cracks or edge roll were observed around the edges of the stretchedacrylic sheet.

As will be apparent from the foregoing, modifications in the presentinvention may be made by those skilled in the art without departing fromthe spirit of the invention. Therefore. this invention is to be limitedonly by the scope of the claims which follow.

I claim:

1. An acrylic blank defined by an upper and a lower surface and by acircumferential end surface and having an initial thickness, saidcircumferential end surface having a circumferential groove thereinhaving a depth at least about 5 percent of said initial thickness ofsaid blank.

2. The acrylic blank of claim 1 wherein said circumferential groove issubstantially coextensive with said circumferential end surface.

3. The acrylic blank of claim 1 wherein said depth of said groove is atleast about 10 percent of said initial thickness of said blank.

4. The acrylic blank of claim 1 wherein said groove is symmetrical abouta plane through the center-line of said circumferential end surfaces.

1. An acrylic blank defined by an upper and a lower surface and by a circumferential end surface and having an initial thickness, said circumferential end surface hAving a circumferential groove therein having a depth at least about 5 percent of said initial thickness of said blank.
 2. The acrylic blank of claim 1 wherein said circumferential groove is substantially coextensive with said circumferential end surface.
 3. The acrylic blank of claim 1 wherein said depth of said groove is at least about 10 percent of said initial thickness of said blank.
 4. The acrylic blank of claim 1 wherein said groove is symmetrical about a plane through the center-line of said circumferential end surfaces. 